The present invention relates to a flexible guide bushing for a machine tool.
This bushing type is designed and used for guiding rods or bars to be machined whose diameter is essentially in a range of from 1 mm to about forty mm on an automatic sliding headstock lathe. It should therefore be differentiated from a collet even though it may exceptionally be used for such an application.
Although the use of rigid guide bushings is still common today, it is not free of problems. One of them is that of the adjustment of the gripping force applied by the bushing to the bar. Indeed, the correct guidance of the latter is decisive for machining quality since an excessive gripping force results in an excessive clamping action that in turn will cause a seizure of the bar, and inversely, an insufficient force will let the bar float, thereby leading to a machining imprecision that increases with the rotational speed of the machined bars. Directly linked to this problem is that of the non-parallelism of the guiding sectors or segments with respect to the axis of rotation on their entire length. Another problem results from the fact that the nominal diameter of a bar is generally not perfectly uniform on its entire length and furthermore the operating range of a given usual bushing is of the order of +0.01 mm to −0.03 mm. The smaller the diameter of the bar, the greater the risk of causing marks on the machined workpieces.
To resolve these problems, flexible guide bushings have been designed.
Currently available on the market is a flexible guide bushing comprising, in a known manner, three sectors that are autonomous as such and connected to each other by elastic members exerting opposed forces. On one hand, outer circular springs serve to maintain the sectors under the action of radial forces directed toward the axis of the bushing. On the other hand, inner compression springs whose axis is orthogonal to the axis of the bushing, i.e. that are arranged tangentially, rest in corresponding seats and serve to spread them apart on their entire length. The whole assembly forms a body that is partly cylindrical and partly frustoconical. After a short operating time, fatigue particularly of the inner springs causes positioning errors of the sectors relative to one another, which errors are amplified by the shocks that may occur when new bars are being loaded. Furthermore, machining chips may pass between the gaps and impair the correct operation of the compression springs. In other words, a precise machining operation will rapidly become impossible since the lifetime of these guide bushings is extremely short.
Another known flexible guide bushing comprises three segments that are interconnected to form a partly cylindrical and partly frustoconical body. A particular bushing can be used for processing bars whose diameter is in a given range. The segments are connected by means of vulcanized rubber in the radial spaces and gaps. It can be said that the stability of this bushing is all but ideal because of the decreasing contact surfaces of the rubber connecting elements with increasing bar diameter and of the non-uniform distribution of the gripping forces applied thereto, more specifically of an absence of force in the middle zone of the bar since the latter is only retained at its ends, which causes are aggravated by a premature deterioration of the connecting elements that is observed during the axial displacement of a bar being machined and due to chips that may remain inside the bushing. The instability of the bushing during machining is a result of its design itself, but above all it will increase over time and thus increasingly prejudice the machining quality.